# std::uninitialized_move_n

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class Size, class NoThrowForwardIt > std::pair<InputIt, NoThrowForwardIt> uninitialized_move_n( InputIt first, Size count, NoThrowForwardIt d_first ); | (1) | (since C++17) (constexpr since C++26)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt > std::pair<ForwardIt, NoThrowForwardIt> uninitialized_move_n( ExecutionPolicy&& policy, ForwardIt first, Size count, NoThrowForwardIt d_first ); | (2) | (since C++17)

1) Copies the elements from first`+` `[`​0​`,` count`)` (using move semantics if supported) to an uninitialized memory area beginning at d_first as if by

for (; count > 0; ++d_first, (void) ++first, --count)
`` ::new (`voidify`(*d_first))
`` typename std::iterator_traits<NoThrowForwardIt>::value_type(/* value */);
return {first, d_first};

where /* value */ is std::move(*first) if *first is of an lvalue reference type, or *first otherwise.
If an exception is thrown during the initialization, some objects in first`+` `[`​0​`,` count`)` are left in a valid but unspecified state, and the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If d_first`+` `[`​0​`,` count`)` overlaps with first`+` `[`​0​`,` count`)`, the behavior is undefined. | (since C++20)

### Parameters

first | - | the beginning of the range of the elements to move
d_first | - | the beginning of the destination range
count | - | the number of elements to move
policy | - | the execution policy to use
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `NoThrowForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `NoThrowForwardIt` may throw exceptions.

### Return value

As described above.

### Complexity

Linear in count.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

When the input iterator deferences to an rvalue, the behavior of `std::uninitialized_move_n` is same as std::uninitialized_copy_n.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms, (1)

### Possible implementation

    template<class InputIt, class Size, class NoThrowForwardIt>
    constexpr std::pair<InputIt, NoThrowForwardIt>
        uninitialized_move_n(InputIt first, Size count, NoThrowForwardIt d_first)
    {
        using ValueType = typename std::iterator_traits<NoThrowForwardIt>::value_type;
        NoThrowForwardIt current = d_first;
        try
        {
            for (; count > 0; ++first, (void) ++current, --count) {
                auto addr = static_cast<void*>(std::addressof(*current));
                if constexpr (std::is_lvalue_reference_v<decltype(*first)>)
                    ::new (addr) ValueType(std::move(*first));
                else
                    ::new (addr) ValueType(*first);
            }
        }
        catch (...)
        {
            std::destroy(d_first, current);
            throw;
        }
        return {first, current};
    }

### Example

Run this code

    #include <cstdlib>
    #include <iomanip>
    #include <iostream>
    #include <memory>
    #include <string>

    void print(auto rem, auto first, auto last)
    {
        for (std::cout << rem; first != last; ++first)
            std::cout << std::quoted(*first) << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::string in[]{"One", "Definition", "Rule"};
        print("initially, in: ", std::begin(in), std::end(in));

        if (constexpr auto sz = std::size(in);
            void* out = std::aligned_alloc(alignof(std::string), sizeof(std::string) * sz))
        {
            try
            {
                auto first{static_cast<std::string*>(out)};
                auto last{first + sz};
                std::uninitialized_move_n(std::begin(in), sz, first);

                print("after move, in: ", std::begin(in), std::end(in));
                print("after move, out: ", first, last);

                std::destroy(first, last);
            }
            catch (...)
            {
                std::cout << "Exception!\n";
            }
            std::free(out);
        }
    }

Possible output:

    initially, in: "One" "Definition" "Rule"
    after move, in: "" "" ""
    after move, out: "One" "Definition" "Rule"

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed
LWG 3918 | C++17 | additional temporary materialization was required when the input iterator deferences to a prvalue | copies the element in this case

### See also

uninitialized_move (C++17) | moves a range of objects to an uninitialized area of memory (function template)
uninitialized_copy_n (C++11) | copies a number of objects to an uninitialized area of memory (function template)
ranges::uninitialized_move_n (C++20) | moves a number of objects to an uninitialized area of memory (algorithm function object)

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